It is usual in powder coating technology to collect excess powder, which does not adhere to the object to be coated, for instance by means of a suction means, in order to recover the powder and re-supply it in a powder cycle to the coating process. In powder coating apparatus, in which coatings of highest quality are to be produced, it is important to know at which ratio fresh and recovered powder is supplied to the supply container of the powder coating guns. A reason for this is the different grain spectra (i.e. the grain size distribution) of the powder particles of fresh and recovered powder. When the suction means operates for instance with a cyclone, as described in the German patent application 197 20 005, the finest portions are filtered out of the powder so that the recovered powder has an average grain spectrum that is coarser than that of the fresh powder. In order to obtain a desired grain spectrum the mixing ratio of fresh and recovered powder should be adjustable.
A relatively complicated method for measuring and regulating this mixing ratio is described in DE 195 81 792 C1. According to this method, fresh and recovered powder is transported at a selected ratio from a main container, or a regenerator container, to a mixing container and is mixed there. The mixing ratio is detected by mathematically predicting the particle size distribution of the powder contained in the mixing container so that the fine portions of the powder in the mixing container do not exceed a predetermined maximum percentage. The determination of the mixing ratio is made on the basis of measurements of the particle size distribution of the pure powder coating material and the particle size distribution of the regenerate.
In a further embodiment described in DE 195 81 792 C1, the pure powder and the excess powder are supplied at a selected ratio from the main container and the regenerate container to the mixing container while the levels in the main container and in the regenerate container do not fall below a minimum level. Additionally, a compensation powder line between the regenerate container and the main container is provided to maintain the desired minimum filling level. The filling levels in the main and regenerate container are detected by means of load cells, which support the container.